1. Field of the Invention
The present invention relates to an integrated circuit such as a monolithic microwave integrated circuit (hereinafter referred to as MMIC) and a method of manufacturing the same. In particular, the present invention relates to an MMIC made of, for example, gallium arsenide (GaAs), capable of providing required capacitance or inductance without increasing chip size, and a method of manufacturing such an MMIC.
2. Description of the Prior Art
GaAs devices achieve both low noise and low distortion characteristics, as well as low power consumption at high frequencies over the UHF band. Thus, GaAs devices have been used widely in TV and CATV tuners and broadcasting satellite receivers. These features are extremely important to mobile communication systems such as cellular cordless phones.
Much concern has been devoted to reducing the mounting area for the components in the mobile communication systems, so as to make the system smaller. So far, many inductors, resistors, and capacitors are placed at the periphery of a GaAs IC, so that the mounting area is increased. An extremely large area will be required for the capacitors or the inductors, if the conventional SiN slab capacitors or spiral inductors are to be integrated in an IC. For that reason, typical GaAs MMICs until now have been supplied with additional pins for these lumped circuit elements. These additional pins end up with increasing the total number of components.
An MMIC made of GaAs or S1 employs, as a bypass capacitor, an MIM (metal-insulator-metal) capacitor consisting of two wiring metal electrodes and an insulation film held between the electrodes. The capacitance of the MIM capacitor is in proportion to the facing areas of the electrodes and in inverse proportion to the distance between the electrodes. To increase the capacitance of the MIM capacitor, therefore, it is necessary to increase the area of each electrode, or thin the insulation film between the electrodes. Increasing the area of each electrode will increase chip size because the MIM capacitor is usually formed flat on the surface of a semiconductor substrate. On the other hand, thinning the insulation film will cause pinholes.
To solve this problem, Japanese Examined Patent Publication No. 5-73273 discloses a technique shown in FIG. 1. An insulation film 102 is grown on a GaAs substrate 101 and is shaped into a stepped pattern. The patterned insulation film 102 is used as a mask to form V-shaped grooves on the substrate 101 by anisotropic etching. Along the grooves and insulation film 102, there are formed a lower electrode 103, an insulation film 104, and an upper electrode 105, to form MIM capacitors. The electrodes of each of the MIM capacitors have large facing areas to provide large capacitance in comparison with a relatively small area occupied by the capacitors on the substrate.
The prior art of FIG. 1, however, increases chip size when it is adopted for a GaAs MMIC because not only active and passive elements but also the patterned insulation film 102 must be arranged on the surface of the substrate 101.
FIG. 2A shows an example of a spiral coil used for an MMIC. The inductance of the coil is approximated as follows: ##EQU1## where n is the number of windings of the coil, a is a mean radius, i.e., a=(dO+di)/4 of the coil, and c=(dO+di)/2. FIG. 2B shows a rectangular spiral coil whose inductance is approximated as follows: EQU L(nH)=8.5nS.sup.5/3 S.sup.1/2 ( 2)
where S is the area (cm2) of the coil. The equations (1) and (2) are, for example, discussed in some detail in chapter 2 of Microwave Components, Devices and Active Circuits, by P. F. Combes, J. Graffeuil and J-F. Sautereau, published 1987 by John Wiley & Sons Ltd. To increase the inductance, the area S of the coil must be increased. Increasing the area of the coil, however, increases the area of the chip. Consequently, the prior art is incapable of providing a small coil with large inductance.
FIG. 3 shows a spiral coil according to a prior art. The coil 56 is made of wiring metal 56a. A center end of the coil 56 is connected, through a contact hole 56b, to an MIM capacitor 57, which is formed on the surface of a semiconductor substrate, to thereby combine conductance L and capacitance C into reactance. The MIM capacitor 57 has a lower electrode layer 57a connected to the ground, an upper electrode layer 57b connected to the coil 56 through the contact hole 56b, and an insulation layer 57c between the electrodes 57a and 57b. The coil and MIM capacitor have each a large area as mentioned above.
FIG. 5 shows a semiconductor substrate of an MMIC. Reactance elements such as those of FIGS. 2A, 2B, and 3 are arranged flat on the substrate. FIG. 4 shows an equivalent circuit of the arrangement of FIG. 5. Coils L.sub.1 and L.sub.2 connected between a power source V.sub.DD and the drains of FETs occupy a large area. Similarly, bypass MIM capacitors C2 and C4 connected between the sources of the FETs and the ground occupy a large area.